1. Field of the Invention
The invention pertains to a radiation reflecting screen which remains efficient and does not deteriorate when used at a determined temperature for a fairly long period of time, especially a reflecting screen which can be used at a temperature of at least 900.degree. C., to the method of its manufacture and to its application to the constitution of a heat-insulation device.
Reflecting screens are used in heat-insulation devices or, again, in heat-concentrating devices.
In heat-insulation devices, the purpose of the reflecting screens is to block heat transmission by infra-red radiation. They are generally used in combination with insulating materials which block the transmission of heat by convection.
2. STATE OF THE PRIOR ART
Thus, light and compact insulating devices are known in the prior art: these devices comprise, for example, thin supports made of mica, lined with a material which has high reflecting power and reduced emissive power, associated with mineral wool and used especially in aeronautics because of their lightness and low bulk.
In heat-concentrating devices, the reflecting screens act as mirrors. Devices of this type are found surrounding furnaces, for example..
The efficiency of a screen is all the greater as its reflecting capacity is high, i.e. when its opaqueness to radiation is substantial. For this, the screens used are made of polished and bright material such as colloidal gold or else molten gold sprayed on the support.
Again, in insulating devices, the efficiency of a screen is all the greater as its emissive power is low. For, depending on the their temperature, certain materials emit an inherent radiation which is often a function of the temperature and which impairs efficiency because it creates heat beyond the screen.
Furthermore, the efficiency of a screen depends on the frequency of the incoming radiation. For a screen may be opaque at certain frequencies and more or less transparent at other frequencies. In this case, the screen may re-emit a portion of the radiation absorbed, in addition to its own temperature-related radiation.
Furthermore, the higher the temperature the more the infra-red radiation received by a screen extends over a very large range of frequencies, and a single screen is not enough to stop the radiation. This is why, in heat-insulation devices used at high temperatures, several screens have to be interposed in between several layers of wool or a similar material in order to stop the radiation to the maximum extent.
Besides, certain materials cannot be used beyond a certain temperature when they are in contact with the ambient air, for they get oxidized and then irreversibly lose all or a part of their reflecting capacity. Consequently, they absorb more radiation and, correlatively, tend to re-emit more radiation.
The same holds true for screens which are interposed in the wool of insulation devices for there is an air cushion in the interior.
In conclusion, the reflecting screens of the prior art behave differently for conditions of use at different temperatures and/or behave differently in relation to incoming frequencies, and cannot be used at high temperatures.